reverted:
--- b/core/lib/Drupal/Core/Entity/ContentEntityFormController.php
+++ a/core/lib/Drupal/Core/Entity/ContentEntityFormController.php
@@ -50,7 +50,7 @@
     $entity = $this->entity;
     // @todo Exploit the Field API to generate the default widgets for the
     // entity fields.
+    if ($entity->getEntityType()->isFieldable()) {
-    if ($entity->getEntityType()->isExtendable()) {
       field_attach_form($entity, $form, $form_state, $this->getFormLangcode($form_state));
     }
 
@@ -138,7 +138,7 @@
     // field API without changing existing entity fields that are not being
     // edited by this form. Values of fields handled by field API are copied
     // by field_attach_extract_form_values() below.
+    $values_excluding_fields = $entity_type->isFieldable() ? array_diff_key($form_state['values'], field_info_instances($entity_type_id, $entity->bundle())) : $form_state['values'];
-    $values_excluding_fields = $entity_type->isExtendable() ? array_diff_key($form_state['values'], field_info_instances($entity_type_id, $entity->bundle())) : $form_state['values'];
     $definitions = $entity->getFieldDefinitions();
 
     foreach ($values_excluding_fields as $key => $value) {
@@ -155,7 +155,7 @@
     }
 
     // Invoke field API for copying field values.
+    if ($entity_type->isFieldable()) {
-    if ($entity_type->isExtendable()) {
       field_attach_extract_form_values($entity, $form, $form_state, array('langcode' => $this->getFormLangcode($form_state)));
     }
     return $entity;
reverted:
--- b/core/lib/Drupal/Core/Entity/EntityFormController.php
+++ a/core/lib/Drupal/Core/Entity/EntityFormController.php
@@ -139,7 +139,7 @@
     $entity = $this->entity;
     // @todo Exploit the Field API to generate the default widgets for the
     // entity properties.
+    if ($entity->getEntityType()->isFieldable()) {
-    if ($entity->getEntityType()->isExtendable()) {
       field_attach_form($entity, $form, $form_state, $this->getFormLangcode($form_state));
     }
 
diff -u b/core/modules/comment/comment.module b/core/modules/comment/comment.module
--- b/core/modules/comment/comment.module
+++ b/core/modules/comment/comment.module
@@ -904,7 +904,7 @@
   // mismatched types. So, we need to verify that the ID is numeric (even for an
   // entity type that has an integer ID, $entity->id() might be a string
   // containing a number), and then cast it to an integer when querying.
-  if ($entity->getEntityType()->isFieldable() && is_numeric($entity->id())) {
+  if ($entity->getEntityType()->isExtendable() && is_numeric($entity->id())) {
     $cids = db_select('comment', 'c')
       ->fields('c', array('cid'))
       ->condition('entity_id', (int) $entity->id())
@@ -956,7 +956,7 @@
   // mismatched types. So, we need to verify that the ID is numeric (even for an
   // entity type that has an integer ID, $entity->id() might be a string
   // containing a number), and then cast it to an integer when querying.
-  if ($entity->getEntityType()->isExtendable() && is_numeric($entity->id())) {
+  if ($entity->getEntityType()->isFieldable() && is_numeric($entity->id())) {
     $cids = db_select('comment', 'c')
       ->fields('c', array('cid'))
       ->condition('entity_id', (int) $entity->id())
reverted:
--- b/core/modules/field/field.api.php
+++ a/core/modules/field/field.api.php
@@ -9,13 +9,13 @@
 use Drupal\field\FieldConfigUpdateForbiddenException;
 
 /**
+ * Exposes "pseudo-field" components on fieldable entities.
- * Exposes "pseudo-field" components on extendable entities.
  *
  * Field UI's "Manage fields" and "Manage display" pages let users re-order
  * fields, but also non-field components. For nodes, these include the title
  * and other elements exposed by modules through hook_form_alter().
  *
+ * Fieldable entities or modules that want to have their components supported
- * Extendable entities or modules that want to have their components supported
  * should expose them using this hook. The user-defined settings (weight,
  * visible) are automatically applied on rendered forms and displayed entities
  * in a #pre_render callback added by field_attach_form() and
@@ -82,7 +82,7 @@
 }
 
 /**
+ * Alter "pseudo-field" components on fieldable entities.
- * Alter "pseudo-field" components on extendable entities.
  *
  * @param $info
  *   The associative array of 'pseudo-field' components.
reverted:
--- b/core/modules/field/field.attach.inc
+++ a/core/modules/field/field.attach.inc
@@ -2,7 +2,7 @@
 
 /**
  * @file
+ * Field attach API, allowing entities (nodes, users, ...) to be 'fieldable'.
- * Field attach API, allowing entities (nodes, users, ...) to be 'extendable'.
  */
 
 use Drupal\Core\Entity\EntityInterface;
@@ -19,11 +19,11 @@
  *
  * Field Attach API functions generally take $entity_type and $entity arguments
  * along with additional function-specific arguments. $entity_type is the type
+ * of the fieldable entity, such as 'node' or 'user', and $entity is the entity
- * of the extendable entity, such as 'node' or 'user', and $entity is the entity
  * itself.
  *
  * An entity plugin's annotation is how entity types define if and how
+ * Field API should operate on their entity objects. Notably, the 'fieldable'
- * Field API should operate on their entity objects. Notably, the 'extendable'
  * property needs to be set to TRUE.
  *
  * The Field Attach API uses the concept of bundles: the set of fields for a
reverted:
--- b/core/modules/field/field.deprecated.inc
+++ a/core/modules/field/field.deprecated.inc
@@ -314,7 +314,7 @@
  *
  * This function performs field validation in the context of a form submission.
  * It converts field validation errors into form errors on the correct form
+ * elements. Fieldable entity types should call this function during their own
- * elements. Extendable entity types should call this function during their own
  * form validation function.
  *
  * @param \Drupal\Core\Entity\ContentEntityInterface $entity
only in patch2:
unchanged:
--- a/core/modules/rest/lib/Drupal/rest/LinkManager/RelationLinkManager.php
+++ b/core/modules/rest/lib/Drupal/rest/LinkManager/RelationLinkManager.php
@@ -87,7 +87,7 @@ protected function writeCache() {
     $data = array();
 
     foreach ($this->entityManager->getDefinitions() as $entity_type) {
-      if ($entity_type->isFieldable()) {
+      if (in_array('isFieldable', get_class_methods($entity_type))) {
         foreach ($this->entityManager->getBundleInfo($entity_type->id()) as $bundle => $bundle_info) {
           foreach ($this->entityManager->getFieldDefinitions($entity_type->id(), $bundle) as $field_definition) {
             $relation_uri = $this->getRelationUri($entity_type->id(), $bundle, $field_definition->getName());